The Colovian barrow set is arguably our most developed and feature-complete dungeon tileset. It has clutter, at least one unique associated creature type, and its own furniture containers. The set itself is interesting and quite different from most Morrowind interior tilesets, although it requires some getting used to. However, there are still a number of assets needed for it to reach its full potential:

Creature types: while we can freely use skeletons and our own ghost creatures, the Barrows could still do with some expansion of their specialty creature, the "Living Mummy". For starters, it needs a different name. Several variations (spearwielder, archer) would also be interesting, as well as a version wearing parts of the Nedic armor set. The Wraith creature would also be useful here.

Armor: the barrow set still needs a featured armor set. Saint_jiub is currently working on this.

Weapons: the set currently has only two weapon types. This makes it uninteresting for players who wield different styles of weapon. I am currently working on expanding this set.

Additional mirrored ceiling tiles should be added to increase versatility.

Entrances and exits currently use the BM dark box door, which is a bothersome model. A dark passage adapted to the shape of the colovian barrow would be very useful.

More rubble and fallen stones would be useful to cover up creative edits.

There is currently only one, rather useless stairs mesh. A type of stairs that go much deeper would be useful, as well as wall and ceiling meshes that conform to it (as well as to the currently existing stairs, which are difficult to use)

Exteriors could use one or two types of collumn or menhir to make them more engaging.

A couple of light sources would also be useful for these interiors, like a large bronze brazier (perhaps based on the clutter for this set)

If we need one thing for this set, it will be maybe 1 or 2 variations of the entrance. Menhirs for the exterior wouldn't hurt either, I agree. I'd be happy to see the ancient nedic armor by Saint_jiub completed, too.

But looking at it in relation to ancient dungeons of neighbouring provinces: For instance Morrowinds comparable dungeon types (velothi towers, dunmer fortresses, ancestral tombs) simply share their dunmer-themed undead and there is no ancient armor set nor any real furniture like ancient tables/closets found. You can give any explanation for it you want, but for me it just means that there is no obligation to really delve so much into each dungeon type. Especially, if we will only be using it in a part of the province.

There is currently only one, rather useless stairs mesh. A type of stairs that go much deeper would be useful, as well as wall and ceiling meshes that conform to it (as well as to the currently existing stairs, which are difficult to use)

You seem to be wanting to use the tileset in a wrong way, if you consider the stairs to be useless. The set was never intended for a great vertical variety or having a really big dungeon in the first place. None of the wall pieces really allow for a sudden shift in verticality, either.

I'm aware that the horizontality of the set was done on purpose, but I am afraid that this is going to limit the applicability of the set in the long run, especially in cases where the exterior landscape is not as accommodating. I'm not proposing a vast array of new pieces that fundamentally change the nature of the set, just one stairs tunnel which can hook up to the entrance pieces. This will allow for some more freedom and versatility, especially when we get to the barrowlands (where I think larger and more complex barrows will be called for).

I don't think the dunmer tombs are the best comparison here, since they are still in use/were in use until not that long ago, which sort of explains why they use common furniture. By comparison, these barrows are by far the oldest locations to be featured. Anyway, custom containers for this set are already finished, so it's kind of a moot point. One or two variations on the mummy are easily made once the nedic armor set is in place - it can be done in nifskope, even.